1. Field
The present invention relates to a pulse transmission method and a pulse transmission system to transmit/receive data by using a pulse of a predetermined pulse width, and to a transmitter and a receiver used for such a transmission system.
2. Description of the Related Art
A pulse transmission is a wired or a wireless transmission using a pulse signal of a very small pulse width as a transmission medium, and is expected to be applied to a wideband radio communication system or the like.
A basic technique of conventional pulse radio transmission, as shown in FIG. 1, is ON/OFF modulation of transmitting information (data) based on presence/absence of a pulse. When time T occupied by a single pulse is called one slot, the presence/absence of a pulse in the slot serves as an information medium. In addition to the ON/OFF modulation, similar binary modulation/demodulation includes a modulation technique of inverting the polarity (+/−) or phase (0°/180°) of a pulse in accordance with data 1 or data 0. When such pulse modulation techniques are used, a maximum bit rate of transmission per unit time is 1/T. Since the necessary frequency bandwidth is 1/T, modulation efficiency defined as a ratio between the maximum bit rate and the necessary frequency bandwidth is 1.
On the other hand, in a carrier radio transmission method using carrier waves instead of pulse signals, multilevel modulation techniques capable of increasing modulation efficiency to the maximum of 2, 4, and 8, respectively, have been adopted, e.g., Quadrature Phase Shift keying (QPSK), 16 Quadrature Amplitude Modulation (16QAM), and 256 Quadrature Amplitude Modulation (256QAM). The pulse radio transmission is requested to increase modulation efficiency to the same extent as carrier radio transmission.
In the pulse radio transmission, a representative multilevel modulation technique that is being discussed to increase modulation efficiency is Pulse Position Modulation (PPM). PPM is described in PCT Japanese Translation Patent Publication No. 2003-521143 and Japanese Unexamined Patent Application Publication No. 2001-111634, for example. PPM is characterized in delaying generation of a pulse by delay time according to a data series. Here, basic delay time which is a basic unit of delay time is called “PPM quantization time”.
FIG. 2 illustrates an example of binary PPM in the case where a pulse slot is divided into eight segments. In PPM, pulse generation time is delayed. Therefore, the time that should be obtained to transmit all data series (symbol length) is extended by the delay time. For example, in PPM where a pulse slot is divided into four segments, it is required to obtain (¾)·T in addition to the pulse slot T in order to transmit four data series ((0, 0), (0, 1), (1, 0), and (1, 1)). Thus, the symbol length is extended to T·(7/4). As a result, a maximum transmission bit rate is 2×(1/T·(7/4))=1.14/T. In this case, improvement in modulation efficiency is smaller relative to the ON/OFF modulation.
In the case of ternary PPM, pulse generation time is delayed by τ×(22·n1+21·n2+n3) with respect to data series (n1, n2, n3). Here, τ=T/23. That is, a pulse slot is divided into eight segments and pulse generation time is changed in accordance with a state of the data series (n1, n2, n3). Thus, the symbol length is extended to T·(15/8) and a maximum transmission bit rate is 3×(1/T·(15/8))=1.6/T. As can be understood, improvement in modulation efficiency is less than double relative to the ON/OFF modulation.
Modulation efficiency of PPM is described below in a more generalized manner. In conventional n-valued PPM, 1 symbol time is defined as the sum of “time T of a Wave Packet” and “total time of a continuous no-signal group (2n−1)·τ” placed before/after time T. Thus, 1 symbol time is expressed by the following expression (1).1 symbol time=T+(2n−1)·τ  (1)
Here “n” represents multiplicity and “τ” represents PPM quantization time. The PPM position number 2n is a total amount of information (=total number of states). Conventional PPM efficiency is expressed by the following expression (2).Modulation efficiency=(n/T+(2n−1)·τ)/(1/T)=n/(1+(2n−1)·τ/T)  (2)
Accordingly, an effective bit rate does not increase for multiplicity with respect to (1/T) in the ON/OFF modulation method.